Adjust installed artifacts for virtual libraries

* Virtual libraries do not install archive files
* Virtual libraries should install .o files

Signed-off-by: Rudi Grinberg <rudi.grinberg@gmail.com>

src/dune_file.ml

@@ -1008,6 +1008,8 @@ module Library = struct
     match t.public with
     | None -> Lib_name.of_local t.name
     | Some p -> snd p.name
+
+  let is_virtual t = Option.is_some t.virtual_modules
 end
 
 module Install_conf = struct

src/dune_file.mli

@@ -249,6 +249,7 @@ module Library : sig
   val dll : t -> dir:Path.t -> ext_dll:string -> Path.t
   val archive : t -> dir:Path.t -> ext:string -> Path.t
   val best_name : t -> Lib_name.t
+  val is_virtual : t -> bool
 end
 
 module Install_conf : sig

src/install_rules.ml

@@ -118,6 +118,7 @@ module Gen(P : Params) = struct
   let lib_install_files ~dir_contents ~dir ~sub_dir ~(name : Lib_name.t) ~scope ~dir_kind
         (lib : Library.t) =
     let obj_dir = Utils.library_object_directory ~dir lib.name in
+    let ext_obj = ctx.ext_obj in
     let make_entry section ?dst fn =
       Install.Entry.make section fn
         ~dst:(
@@ -148,21 +149,24 @@ module Gen(P : Params) = struct
         in
         Module.Name.Map.values modules
       in
+      let virtual_library = Library.is_virtual lib in
       List.concat
         [ List.concat_map modules ~f:(fun m ->
             List.concat
               [ [ Module.cm_file_unsafe m ~obj_dir Cmi ]
               ; if_ (native && Module.has_impl m)
                   [ Module.cm_file_unsafe m ~obj_dir Cmx ]
+              ; if_ (native && Module.has_impl m && virtual_library)
+                  [ Module.obj_file m ~obj_dir ~ext:ext_obj ]
               ; List.filter_map Ml_kind.all ~f:(Module.cmt_file m ~obj_dir)
               ; List.filter_map [m.intf;m.impl] ~f:(function
                   | None -> None
                   | Some f -> Some f.path)
               ])
-        ; if_ byte [ Library.archive ~dir lib ~ext:".cma" ]
-        ; if_ (Library.has_stubs lib)
+        ; if_ (byte && not virtual_library) [ Library.archive ~dir lib ~ext:".cma" ]
+        ; if_ (Library.has_stubs lib && not virtual_library)
             [ Library.stubs_archive ~dir lib ~ext_lib:ctx.ext_lib ]
-        ; if_ native
+        ; if_ (native && not virtual_library)
             (let files =
                [ Library.archive ~dir lib ~ext:".cmxa"
                ; Library.archive ~dir lib ~ext:ctx.ext_lib